Excise "exclude" column from meta attribute

[danielhuppmann]

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• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

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- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

This PR excises the "exclude" column from the "meta" attribute to reduce confusion among users why an exclude column is added (but then again not imported to the IIASA Scenario Explorer.

The PR:

• creates a new attribute exclude and refactors all validation methods such that the argument exclude_on_fail changes this attribute
• extends the filter such that filter(exclude=<value> works as expected
• adds "exclude" to the list of illegal column names for (timeseries) data and meta
• when importing an xlsx file created with pyam < 2.0 (which has an "exclude" column in "meta"), that column is moved to the new exclude attribute (with a log message)
• streamlines a number of tests

fyi @gidden @byersiiasa @phackstock

closes #755

[danielhuppmann]

Wrong head branch...